The ultimate aim of these studies is to develop potent antiestrogens capable of controlling growth of estrogen-dependent tumors in humans by non-invasive endocrine therapy. Because of their known specific interaction with estrogen receptors, antiestrogens seem particularly well suited to be specific, antitumor agents in the cases of estrogen dependent tumors, theoretically capable of achieving non-surgical endocrine therapy. Our approach is to understand the mechanistic basis of the antagonist action of these compounds and how this stems from their interaction with estrogen receptors in target tissues and their in vivo pharmacokinetic behavior. Studies will utilize unlabeled antiestrogens, and antiestrogens in radiolabeled form which we have prepared, to study their in vivo biotransformations and to identify the structure of the antiestrogen metabolites active in vivo; and to probe the status, levels, and physicochemical characteristics of receptor-antiestrogen vs. receptor-estrogen complexes and the nature of their interaction in the nucleus. Studies will be further advanced in rat uterus and dimethylbenzanthracene (DMBA)-induced mammary tumors, and will utilize a spectrum of mammary tumor systems (DMBA, R323OAC, and 13762E) and several human breast tumor cell lines displaying varying degrees of estrogen responsiveness and dependence as models for the apparent spectrum of hormone responsiveness of human breast cancers. In these systems, we will explore the possible involvement of prolactin, and will analyze antiestrogen-sensitive and resistant breast cancer cloned cell lines to explore the basis of sensitivity and resistance to antiestrogen-mediated growth inhibition and cell killing. Comparative studies will be done in normal mammary tissue and cells, and in the liver and pituitary in which estrogens evoke distinct biological responses. These studies should elucidate the mechanistic bases by which antiestrogens antagonize the growth of estrogen-dependent tissues and tumors and should enable the clinical use of antiestrogens to be approached with greater rationale.